1. Field of the Invention
The present invention relates to a method of forming a glass product having a smooth surface and, more particularly, to a method of forming a deep-draw glass product partially having a smooth surface such as a display front panel of a low-profile television.
2. Description of the Prior Art
In a method of forming a front panel of a cathode-ray tube, a molten glass gob is press-molded to a predetermined shape, and a portion required to have a smooth surface of the molded piece is polished. Alternatively, a glass plate is placed and heated on a mold, and is then subjected to vacuum forming.
However, of these conventional methods, in a method of press-molding a gob, since a molten glass gob is pressed against the surface of a mold upon molding using the mold, impressions remain on the surface of a glass product. For this reason, a portion required to have a smooth surface must be polished after molding. As a result, the number of steps is increased, productivity is impaired, and cost is increased.
In a vacuum forming method, since a force contributing to deformation of a glass plate is only a vacuum force, a portion to be deformed of the glass plate must be kept at a high temperature. When vacuum forming is performed in such a state, the thickness of the portion to be deformed at a high temperature is considerably decreased. In addition, impressions due to contact with a mold are formed on a glass plate portion near the portion to be deformed.
In the vacuum forming method, or in a method wherein a glass plate is molded while being clamped between male and female molds, when a glass plate is placed and heated on the female mold, a flat portion of the glass plate is inevitably deflected due to its weight.
On the other hand, in a method wherein a glass plate is clamped between male and female molds to heat and mold it, a sharing force always acts on the glass plate. For this reason, if deep drawing is performed, a glass plate portion defining a side surface is expanded, and its thickness is decreased, thus posing a problem of mechanical strength. Therefore, this method can only be applied to molding of small products such as a fluorescent tube.
As a problem common to these methods wherein a glass plate is re-heated and molded, thermal cracking during cooling or deflection after cooling easily occurs due to temperature nonuniformity at the beginning of cooling. In order to solve this problem, when a glass plate is entirely heated to the same temperature as that of a portion to be deformed, the entire surface of the molded piece suffers from deflection or impressions due to heat.